There is extensive research in developing selective and sensitive gas sensors for applications spanning an incredible range of technologies, from environmental, energy optimization, food, health and security. Just as the applications are varied, so are the types of gas sensing technologies that are being developed, with major advances being made in optical and electrochemical devices. There have been some remarkable successes, for example the ubiquitous electrochemical oxygen sensor for combustion control as well as fire detection sensors. The challenges in this field continue to be developing selectivity and sensitivity with respect to specific gases present in harsh environments, and reducing the foot print of the sensing device and measurement system.
A good example of an unmet sensing need is control of NO emissions for transportation systems running on diesel fuel. In the after treatment system for reducing NO emissions, NO sensors are required with discrimination against other combustion gases, and capable of operation under very harsh environments. NO sensors are also required for breath analysis for diagnosis of respiratory diseases. The most common measurement technique in breath monitoring is the chemiluminescence analyzer, however this apparatus is bulky, and requires a supply of ozone. There is considerable interest in developing minaturizable electrochemical sensors that have high sensitivity (ppb level) to NO and yet can discriminate against hundreds of other molecules in breath.
Metal oxide semiconductors (MOS)-based sensors (n-type: SnO2, ZnO2, TiO2, WO3, etc.; p-type: CuO, Cr2O3, etc.) may be used to detect volatile compounds (such as acetone, propanol, ethanol) or toxic gases (such as CO, NO, NO2, etc.). In general terms, MOS sensors incorporate a sensing layer formed of material selected for a targeted gas. When the targeted gas interfaces with the sensing layer material, the target gas molecules are adsorbed on the crystal surface, resulting in a change in conductivity of the sensing layer. By measuring the change in conductivity (e.g., resistivity), the presence and amount (often in ppm or ppb) of the targeted gas (or other compound or analyte of interest) can be estimated. Sensitivity/selectivity to a particular gas depends on the intrinsic properties of the MOS material, and can be modulated by doping to alter the electrical properties or by introducing catalysts such as Au, Pt, Pd to alter the chemical properties.
The concept of using p and n-type semiconducting oxide (MOS) as well as their mixtures to improve sensor performance is reported in the literature. For the mixtures of p and n-type materials, there are primarily two strategies, mixing p- and n-type powders or creating a p-n diode-type junction.
For particular ratios of powder mixtures of n-type anatase and p-type rutile, it has been found that the resistance change is minimal towards CO and CH4. Based on a polychromatic percolation model, it was proposed that at these particular powder mixture ratios, the two parallel conduction pathways based on n-n and p-p paths cancel each other. Other studies have noted similar effects, for mixtures of ZnO (n-type) and Al-doped CuO (p-type) increasing CuO exhibited lower response to CO. In another study, Pt loading on mixtures of n-type ZnO and p-type CuO, led to an overall p-type response towards CO. CO selectivity was also noted for CuO/ZnO heterocontacts. Other strategies have been to put p-type Co3O4 nanoparticles on n-ZnO nanowires, as well as nanocomposites and p-type CuO on n-type SnO2 nanorods exhibited high sensitivity to H2S.
There are several studies in the literature focused on gas sensing which report the formation p-n junctions that exhibit I-V characteristics indicative of rectification. These include n-ZnO/p-CuO, Pt/SnO2/n-Si/P+—Si/Al, p-ZnO/n-ZnO, ZnO (p-type)/n-Si heterojunctions, and n-SnO2/p-Co2O3 (or Cr2O3).
Still other studies have demonstrated that both WO3 and Cr2O3 exhibit an increase in resistance upon exposure to NO, whereas in the presence of CO, there is an increase in resistance for Cr2O3 and a decrease in resistance with WO3.
In light of the above, a need exists for sensors, and related sensor systems, for sensing NO in various environments, including human breath.